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DETERMINATION OF MOLECULAR WEIGHT         35
the cork and weigh the tube and benzene. Fill up the outer jar
with water and small lumps of ice and stir from time to time.
Whilst the benzene is cooling in the apparatus the Beckmann
thermometer may be adjusted.
Adjustment of the Beckmann Thermometer.-
Determine first the value of the mercury thread in degrees
between the top of the scale and the orifice of the reservoir.                    tr<
This may be clone by warming the bulb in a water-bath along                    \\%{
with an ordinary thermometer.    As soon as sufficient mercury                  '
has collected at the orifice, the burner is removed, the water                  ' ^i|
well stirred, and the little bead of mercury detached by gently
tapping the head of the thermometer without removing the bulb
from the water. The temperature on the ordinary thermometer                   /f|
is noted and is again read off when the mercury in the Beck-                    v'!/
mann thermometer has subsided to the top of the scale.     Sup-                    |\ ^
posing, then, the value of the thread above the scale to have                   '',/*';
iDeen determined and equivalent to 2, and the freezing-point of                    i]/
benzene to be about 4, the thermometer degrees may in this case                     \ *
be made to coincide with the Beckmann degrees, which will bring                        >l'f "*
the thread of mercury well up the scale.    The bulb of the thermo-                      ^
meter will therefore require to be at a temperature of 64-2 = 8                      $\
before removing the excess of mercury.    It will, however, be                       '1
necessary to introduce more mercury into the bulb.    This  is                        I
done by inverting the thermometer and tapping it gently on the                       ' ]
palm of the hand, so as to detach a bead of mercury, which                    * J
slips down to the orifice of the capillary. By warming the bulb
the mercury is driven to the top and coalesces with that in the
reservoir, so that on cooling the additional mercury runs into
the bulb. When sufficient mercury has been added the thermo-
meter is cooled to 8, and the excess detached as described above
The zero should now coincide approximately with that of ice-
cold water. If the thermometer is to be adjusted to any other
temperature it is placed in water and warmed to that tempera-
ture -h the number of degrees on the scale above that point
4- the value of the thread above the scale. The excess, of
mercury is then detached. The thermometer being adjusted,
insert it through the cork so that the bulb is well covered by the
benzene, and let the benzene cool well below its freezing-point
before stirring. Tap the head of the thermometer occa-
sionally with a pencil. Now stir briskly for a moment As soon
D 2